Prion protein dimers useful for vaccination

ABSTRACT

Described is a prion protein, wherein said prion protein is a homodimer or heterodimer. The prion protein dimers are highly immunogenic capable of inducing an immune response in a mammal, thus useful for prophylactic or therapeutic vaccination against diseases associated with the infectious forms of prion proteins, PrP Sc , i.e. transmissible spongiform encephalopathies. Moreover, antibodies generated by using the prion protein diners as an antigen, pharmaceutical composition containing the prion protein dimers or antibodies directed against said diners as well as DNA sequences encoding the prion protein diners are described.

This application claims priority under 35 U.S.C. §§ 119 and/or 365 EP 01109707.8 filed Europe on 19 Apr. 2001; the entire content of which is hereby incorporated by reference.

The present invention relates to a prion protein, wherein said prion protein is a homodimer or heterodimer. The prion protein dimers are highly immunogenic capable of inducing an immune response in a mammal, thus useful for prophylactic or therapeutic vaccination against diseases associated with the infectious forms of prion proteins, PrP^(Sc), i.e. transmissible spongiform encephalopathies. Furthermore, the present invention relates to antibodies generated by using the prion protein dimers as an antigen, pharmaceutical compositions containing the prion protein dimers or antibodies directed against said dimers as well as DNA sequences encoding the prion protein dimers.

Transmissible spongiform encephalopathies are neurodegenerative diseases such as scrapie of sheep, bovine spongiform encephalopathy (BSE) of cattle and Creutzfeldt-Jakob disease (CJD) of man. Infectious preparations derived from infected brains are resistant to ultraviolet and ionizing radiation as well as other procedures which normally inactivate nucleic acids indicating that nucleic acids are not required for infectivity. Purification of infectious preparations from brains revealed the presence of a protein required for infectivity. These experimental observations led to the “protein only” hypothesis which proposes that particular proteinaceous infectious particles (prions) are responsible for the transmission of transmissible spongiform encephalopathies (Prusiner et al., Proc. Natl. Acad. Sci. USA 95, 13363-13383 (1998)). Prions consist mainly of a protease resistent protein designated PrP^(Sc) (prion protein, “Sc”: scrapie), which is an abnormal isoform of the proteinase K sensitive PrP^(C) (“C”:cellular). The prion protein PrP^(C) is a species specific protein which is physiologically expressed in brain cells and peripheral blood cells as well as, e.g., in spleen. The biological function of PrP^(C) is so far unknown. Transgenic mice which are no longer capable of expressing PrP^(C) (PrP knock out mice) do not show any pathological damages and cannot be infected with prions (Bueler et al., Nature 356, 577-582 (1992); Bueler et al., Cell 73, 1339-1347 (1993)).

Both isoforms, PrP^(Sc) and PrP^(C), share the same amino acid sequence, but differ in their secondary structure. Circular Dichroism (CD) and Fourier Transform Infrared (FTIR) spectroscopy revealed a significantly higher β-sheet content for PrP^(Sc) as compared to a high α-helix content in PrP^(C). It has been suggested that prion propagation involves the conversion of α-helical domains in PrP^(C) into β-sheets in PrP^(Sc). The in vitro conversion of PrP^(C) into a PrP^(Sc)-like molecule was demonstrated employing a proteinase K resistance assay.

It has been suggested that for the development of a transmissible spongiform encephalopathy the interaction of PrP^(Sc) and PrP^(C) is a crucial event, wherein PrP^(Sc) forces a conversion of PrP^(C) into the pathological conformation (PrP^(Sc)) Accordingly, transgenic mice which are no longer capable of expressing PrP^(C) (PrP knock out mice) are resistant to infections with PrP^(Sc). Although, the prion proteins of various species differ as regards their amino acid sequences, within the group of mammals there is a high homology (88 to 99%). Apparently, due to differences in the amino acid sequences the prion protein cannot be transmitted between certain species (relative or absolute species barrier). Due to the fact that the prion protein is a physiologically expressed protein, each species exhibits immunological tolerance via its own specific prion protein. Unfortunately, so far transmissible spongiform encephalopathies, e.g. BSE, are incurable. There is no therapy available for soothing the symptoms of the disease, let alone for stopping the disease, e.g. by vaccination.

Therefore, it is the object of the present invention to provide a vaccine for the prevention or treatment of a transmissible spongiform encephalopathy.

According to the invention this is achieved by the subject matters defined in the claims.

The present invention provides a prion protein, wherein said prion protein is a homodimer or heterodimer with the monomer corresponding to or comprising a prion protein according to the definition below. During the experiments leading to the present invention, it has, surprisingly, been found that prion protein dimers are useful for inducing an enhanced immune response against PrP^(c) and PrP^(Sc). Using this apparent remarkable immunogenicity of the prion protein dimer it was even possible in vivo to generate antibodies directed against the homologous prion protein (i.e. mouse antibodies directed against murine PrP), thereby abolishing or by-passing the known anto-tolerance present within a given species. After incubation of cells permanently producing PrP^(Sc) with the antibodies that were generated by immunization with the prion protein dimer for 16 hours a drastic inhibition of the de novo synthesis of PrP^(Sc) could be observed. Presumably, this result is due to generation of a functional PrP^(c) knock out state where the surface-located PrP^(c) substrate needed for continuous cellular conversion into PrP^(sc) is functionally impaired, resulting finally in inhibition of generation of PrP^(sc). It is known from the literature that the surface location of PrP^(c) is a prerequisite for cellular prion conversion (Taraboulos et al., J. Cell Biol. 129, 121-132 (1995)). Even more, studies with transgenic mice have demonstrated that peripheral expression of PrP^(c) is absolutely required for replication and transport of PrP^(sc) from peripheral sites of the body to the central nervous system (Blättler et al., Nature 389, 69-73 (1997)). In this respect it has been discovered by the inventors that after application of the prion protein dimer of the present application to a subject, the induced immune response results in the downregulation of the surface expression of PrP^(c). Accordingly, the conversion rate (PrP^(c)

PrP^(Sc)) can be drastically reduced, thus, blocking the initiation and the progression of the disease. Thus, the prion protein dimers of the present invention are useful as a vaccine for the prevention or treatment of a transmissible spongiform encephalopathy.

Accordingly, the present invention relates to a prion protein, wherein said prion protein is a homodimer or heterodimer which can elicit an immune response against PrP^(C) or PrP^(Sc), preferably an immune response which is higher compared to the immune response obtained using for vaccination the corresponding monomer. As used herein, the term “prion protein” comprises the native (full length and/or mature) protein as well as variants thereof, e.g. truncated versions, which are still useful for eliciting an enhanced immune response, i.e. still useful for vaccination. In this context, the term “protein” also comprises peptides or polypeptides having a length of at least 8, preferably of at least 15 and, more preferably, of at least 20 amino acids. The person skilled in the art knows sources for the prion protein monomers and DNA sequences encoding the prion protein monomers, respectively (Schätzl et al., J. Mol. Biol. 245, 362-374 (1995); Wopfner et al., J. Mol. Biol. 289, 1163-1178 (1999); sequences deposited at Genbank accessible via Public Medline database). Furthermore, a person skilled in the art knows methods for coupling the monomers in order to obtain the dimer. Preferably, the monomers are covalently coupled.

In a preferred embodiment, the homodimer or heterodimer is a fusion protein comprising the monomeric prion protein units. Such a fusion protein can be generated by methods well known by the person skilled in the art. The selection of a particular method mainly depends on the length of the monomers, i.e. very short fusion proteins are preferably chemically synthesized, e.g. by standard solid phase synthesis, whereas longer fusion proteins are preferably produced by expression of the corresponding DNA sequences in a suitable host and isolation/purification of the fusion protein from the host or the culture medium.

Generally, by means of conventional molecular biological processes (see, e.g., Sambrook et al., 1989, Molecular Cloning, A Laboratory Manual, 2^(nd) edition Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.) it is not only possible to generate a DNA sequence encoding the desired fusion protein but, additionally, to introduce different mutations into the DNA sequence. One possibility is the production of deletion mutants in which DNA molecules are produced by continuous deletions from the 5′- or 3′-termini of the coding DNA sequence and that lead to the synthesis of proteins that are shortened accordingly. The person skilled in the art can easily check as to whether such prion protein diners containing shortened monomeric units are still useful as a vaccine, e.g. using the methods described in the examples, below. For the manipulation in prokaryotic cells by means of genetic engineering the DNA sequences can be introduced into plasmids allowing a mutagenesis or a modification of a sequence by recombination of DNA sequences. By means of conventional methods (cf. Sambrook et al., 1989, Molecular Cloning: A Laboratory Manual, 2^(nd) edition, Cold Spring Harbor Laboratory Press, N.Y., USA) bases can be exchanged and natural or synthetic sequences can be added. Furthermore, manipulations can be performed that provide suitable cleavage sites or that remove superfluous DNA or cleavage sites. If insertions, deletions or substitutions are possible, in vitro mutagenesis, primer repair, restriction or ligation can be performed. As analysis method usually sequence analysis, restriction analysis and other biochemical or molecular biological methods are used.

In a more preferred embodiment, the monomers of the fusion protein are linked via a peptide spacer. Suitable peptide spacers, which preferably consist of non-polar and non-bulky amino acids, have a length below 20 residues, provide a certain flexibility, thereby avoiding e.g. cystein or prolin residues which can induce rigidity (or vice versa in cases when rigidity or induction of an orientation/turns are wanted). These spacers are known to the person skilled in the art and comprise for example AGAIGGA [HIV-1 protease; Kräusslich et al., Proc. Natl. Acad. Sci. USA, 88, 3212-3217 (1991)], SGGRGG [HIV gp41; Tan et al., Proc. Natl. Acad. Sci USA, 94, 12303-12308 (1991)], GSGGGGSGGGGSGGSGA [single chain T-cell receptor; Khandekar et al., J. Biol. Chem. 272, 32190-32197 (1997)] or (GGGGS)_(n=1-3) [single chain antibodies; Kortt et al., Eur. J. Biochem. 221, 151-157 (1994)]. Examples of preferred spacers are (a) spacers having a length of 2 to 3 amino acids, wherein the DNA sequence encoding said spacer contains a restriction site, (b) oligomers composed of small neutral amino acids (e.g. Ala-Gly-Ala-Ile-Gly-Gly-Ala), or c) an oligomer having an amino acid sequence that defines an epitope, e.g. flag-tag, HA-epitope etc. . . .

For the generation of the prion protein dimers encoding DNA sequences and for constructing expression vectors which contain said DNA sequences, it is possible to use general methods known in the art. These methods include e.g. in vitro recombination techniques, synthetic methods and in vivo recombination methods as described in Sambrook et al., Molecular Cloning, A Laboratory Manual, 2^(nd) edition (1989) Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., for example.

For recombinant production of the dimeric prion protein of the invention, the DNA sequences encoding said protein are inserted into a suitable expression vector. Preferably, they are plasmids, cosmids, viruses, bacteriophages and other expression vectors usually used in the field of genetic engineering. Expression vectors suitable for use in the present invention include, but are not limited to e.g. pGEMEX, pUC derivatives, pGEX-2T, pET3b, T7 based expression vectors and pQE and pTrcHis (for expression in E.coli). For the expression in yeast, e.g. pY100 and Ycpad1 have to be mentioned while e.g. pcDNA3.1, pKCR, pEFBOS, cDM8, pMSCND, and pCEV4 have to be indicated for the expression in animal cells. The baculovirus expression vector pAcSGHisNT-A is especially suitable for the expression in insect cells. The DNA sequences can also be contained in a recombinant virus containing appropriate expression cassettes. Suitable viruses that may be used in the present invention include baculovirus, SFV, vaccinia, sindbis virus, SV40, Sendai virus, adenovirus, an AAV virus or a parvovirus, such as MVM or H-1. The vector may also be a retrovirus, such as MOMULV, MoMuLV, HaMuSV, MuMTV, RSV or GaLV. Preferably, the DNA sequence is operatively linked to the regulatory elements in the recombinant vector that guarantee the transcription and synthesis of an RNA in prokaryotic and/or eukaryotic cells that can be translated. The nucleotide sequence to be transcribed can be operably linked to a promoter like a T7, metallothionein I or polyhedrin promoter. For expression in mammals, the DNA sequences are operatively linked to a suitable promoter like, e.g. a human cytomegalovirus “immediate early promoter” (pCMV).

Suitable host cells include bacteria, yeast, insect and animal cells, preferably mammalian cells. The E. coli strains HB101, DH1, x1776, JM101, JM109, BL21, XL1Blue and SG 13009, the yeast strain Saccharomyces cerevisiae, the insect cells sf9 and the animal cells L, A9, 3T3, FM3A, BHK, human SW13, CHO, COS, Vero and HeLa are preferred. Methods of transforming these host cells, of phenotypically selecting transformants and of expressing the DNA according to the invention by using the above described vectors are known in the art. For recombinant production of the dimeric prion protein of the invention, the host cell is cultivated under conditions allowing the synthesis of the protein and the protein is subsequently isolated from the cultivated cells and/or the culture medium. Isolation and purification of the recombinantly produced proteins may be carried out by conventional means including preparative chromatography and immunological separations involving affinity chromatography with monoclonal or polyclonal antibodies, e.g. with the antibody A7 and 3F4 described in the examples, below.

In an alternative preferred embodiment the monomers of the dimeric prion protein of the invention are covalently coupled via suitable compounds, e.g. in such a way that the monomers are covalently linked via their C-termini or N-termini or via the C-terminus of one monomer and the N-terminus of the other monomer. Care has to be taken that dimerization is carried out in such a way that eliciting an immune response by the dimeric protein is not adversely effected. Suitable compounds for dimerization are known to the person skilled in the art, preferably such a compound is a polyethylenglycole, activated benzodiazepine, oxazolone, aminimide, azalactone, diketopiperazine or a monosaccharide. The corresponding chemical reactions are also known to the person skilled in the art.

The monomers of the dimeric prion protein of the present invention can be identical (homodimer: the monomers are derived from the same species) or different (heterodimer: the monomers are derived from different species). In the case of the heterodimer it can be expected that an immune response against the own PrP^(C) as well as the foreign PrP^(C) is induced.

As already described above the dimeric prion protein of the present invention also includes monomers which are variants of the native prion protein monomers. Particularly preferred are variants which are N-terminally and/or C-terminally truncated proteins (monomers).

A preferred dimeric prion protein (and its corresponding nucleotide sequence) is shown in FIG. 5.

The present invention also relates to the use of a dimeric prion protein of the present invention for the production of polyclonal or monoclonal antibodies as well as to an antibody, preferably a monoclonal antibody which binds to the dimeric prion protein of the present invention. Such antibodies are useful for passive immunization and can be produced according to well known methods. The term “antibody”, preferably, relates to antibodies which consist essentially of pooled monoclonal antibodies with different epitopic specificities, as well as distinct monoclonal antibody preparations. Monoclonal antibodies are made using as an antigen the dimeric prion protein of the invention by methods well known to those skilled in the art (see, e.g., Köhler et al., Nature 256 (1975), 495). As used herein, the term “antibody” (Ab) or “monoclonal antibody” (Mab) is meant to include intact molecules as well as antibody fragments (such as, for example, Fab and F(ab′)2 fragments) which are capable of specifically binding to protein. Fab and F(ab′)2 fragments lack the Fc fragment of intact antibody, clear more rapidly from the circulation, and may have less non-specific tissue binding than an intact antibody. (Wahl et al., J. Nucl. Med. 24:316-325 (1983).) Thus, these fragments are preferred, as well as the products of a FAB or other immunoglobulin expression library. Moreover, antibodies of the present invention include chimeric, single chain (scFv), and humanized antibodies.

Moreover, the present invention also relates to DNA sequences encoding the dimeric prion proteins of the present invention as well as an expression vector containing such a DNA sequence. Examples of such DNA sequences and suitable expression vectors are already described above. Such DNA sequences or expression vectors are not only useful for the recombinant production of the dimeric prion protein of the present invention but also useful as “genetic vaccine”. For the preparation of a genetic vaccine, the DNA sequences of the invention are inserted into an appropriate vector under the control of a suitable promoter, e.g. a vector or promoter as described above. Additional suitable vectors are, e.g., herpes simplex virus type 1 amplicon or recombinant Semliki forest virus vectors, plus-strand RNA virus vectors (lentiviral vectors), minus-strand RNA virus vectors (e.g. Rabies virus, RSV). In this connection also “gene gun” applications are suitable. A vaccine according to the present invention can be used to stimulate humoral and/or cellular immune response in subjects who may benefit from such responses by protection against or treatment of a transmissible spongiform encephalopathy.

Finally, the present invention relates to a pharmaceutical composition comprising a dimeric prion protein, an antibody, DNA sequence or an expression vector of the present invention in a pharmaceutically acceptable carrier. After application to the subject, the immune response induced should result in the downregulation of the surface expression of PrP^(C). Accordingly, the transport of infectious prions from peripheral sites of inoculation (e.g. oral or i.v.) to the central nervous system as well as the initial peripheral replication should be significantly impaired. In addition, the conversion rate (PrP^(C)

PrP^(Sc)) can be drastically reduced, thus blocking the progression of the disease. Thus, the compounds of the present invention are useful for the preparation of a vaccine for the prevention or treatment of a transmissible spongiform encephalopathy (prophylactic or therapeutic vaccination). For the preparation of the vaccine the purified compounds described above may be combined with any suitable adjuvant such as ISCOMS, alum, Freund's Incomplete or Complete Adjuvant, Quil A and other saponins or any other adjuvant as described in the literature. Suitable doses and routes of administration depend on the particular compound and are well known to the person skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: PrP dimer

Coomassie blue staining after purification showing a purity of >90%. M, molecular weight standards.

FIG. 2: Characterization of the PrP dimer using CD

The results show that the PrP monomer and the PrP dimer have a similar or even identical secondary structure.

FIG. 3

(a) Immunological responses in different mice

Immunization with the PrP monomer and PrP dimer, respectively. Recombinant murine PrP was separated on a SDS PAGE gel, subsequently transferred to a PVDF membrane and the immunoblots were probed with decreasing concentrations auf mice antisera. The immunization with the PrP dimer resulted in an enhanced antibody production.

(b) Immunological responses in different rabbits

Immunization with the PrP monomer and PrP dimer, respectively. Recombinant PrP monomer (PrP_(M)) and PrP dimer (PrP_(D)), respectively, were separated on a SDS PAGE gel, subsequently transferred to a PVDF membrane and the immunoblots were probed with anti-PrP_(monomer) and anti-PrP_(dimer), respectively. The immunization with the PrP dimer resulted in an enhanced antibody production.

FIG. 4: In vitro experiments using PrP^(Sc) infected cells

During incubation with antibodies for 16 hours proteins were metabolically labeled using ³⁵S-methionine and subsequently immunoprecipitated (RIPA). Prior to the RIPA the cell lysates were separated into two fractions for digestion with proteinase K (+/−) and separated into soluble and insoluble fractions using ultracentrifugation (1 hour, 100,000×g, 1% sarcosyl); data only shown for the insoluble fraction. Only after incubation with A7 a drastic decrease of the de novo synthesis of PrP^(Sc) can be observed. A,B: after deglycosylation using PNGase F; K: without incubation with antibodies; P: preimmune serum; 3F4: monoclonal antibody directed against the 3F4 epitope of the prion protein; A7: antibody obtained from rabbits which had been immunized with the PrP dimer.

FIG. 5: Translation of hisPrPDPcr (1-1320)

Underlined in the first line: His-Tag

Underlined in the middle: Linker

The present invention is explained by the examples.

EXAMPLE 1 Generation of a Fusion Protein (Dimer) Comprising Two Prion Protein Monomers Covalently Linked by a Peptide Linker

Mouse genomic DNA encoding the prion protein (murine PrP-A, derived from murine N2a neuroblastoma cell line; for sequence see Schätzl et al, 1995, supra; Wopfner et al., 1999, supra) was amplified by polymerase chain reaction (PCR) using the following primer pairs which introduce a peptide linker (Ala-Gly-Ala-Ile-Gly-Gly-Ala) encoding sequence containing a PvuI restriction site; primer pair I: 5′-GCG GAT CCG TCG CCA CCA TGG CGA ACC TTG GCT A-3′ (PrPB+1) and 5′-ACC GAT CGC TCC AGC GCT GGA TCT TCT CCC GTC GTA AT-3′ (231+L new); primer pair II: 5′-AGC GAT CGG TGG AGC TAA AAA GCG GCC AAA GCC TGG AG-3′ (23+L new); 5′-ATC TAG ATC ATC ATC ATC CCA CGA TCA GGA AGA-3′ (254 PX) (the DNA sequence encoding the PvuI restriction site is underlined). For the introduction of an N-terminal His-tag, the removal of the N-terminal and C-terminal signal peptides and the introduction of additional restriction sites, a second PCR was carried out using the following primers: 5′-GAT GTT GGA TCC TGC AAG AAG CGG CCA AAG-3′ (5′-primer, the BamHI-site is underlined.); 5′-GGA GGA GAT CCA GCA GCT AGA AGC TTT TC-3′ (3′-primer, the HindIII-site is underlined.) The PCR fragment obtained (c.f. FIG. 5) was inserted into the pQE30 expression vector (Qiagen, Hilden, Del.). In order to obtain higher yields of the recombinantly produced protein, proteinase deficient bacteria (BL21, Stratagene Europe, Amsterdam, The Netherlands) were used for expression. Four hours after induction of expression using 2 mM IPTG, bacteria were lysed in 6 M guanidine hydrochloride (6 M guanidine hydrochloride, 20 mM sodium phosphate, 500 mM sodium chloride, pH 7,8). cell debris were pelleted by centrifugation (20 min., 10,000×g), the supernatant was loaded on a Ni²⁺loaded SP-HiTrap column (Amersham Pharmacia, Uppsala, Sweden) which had been previously equilibrated using binding buffer (8 M urea, 20 mM sodium phosphate, 500 mM sodium chloride, pH 7.8) and incubated. The column was washed for several times (8 M urea, 20 mM sodium phosphate, 500 mM sodium chloride, 80 mM imidazole, pH 6.3) and, subsequently, the fusion protein containing the His-tag was eluted using 8 M urea, 20 mM sodium phosphate, 500 mM sodium chloride, 500 mM imidazole, pH 6.3. Fractions of the eluate were collected. The fractions showing the highest protein concentration were pooled and for removal of urea dialyzed against pure water (Slide-A-Lyzer, Pierce, Bonn, Del.). As shown in FIG. 1, the protein could be obtained with a very high purity. As shown in FIG. 2, the secondary structure of the purified renatured dimer is similar (or even identical) to the secondary structure of the prion protein monomer.

EXAMPLE 2 Immunization of Mice and Rabbits

Ten female B6D₂F₁ (B57BL/6×DBA) mice (inbred strain generated at the Institute of Molecular Animal Breeding/Gene Center Munich, Del.; six weeks old) were immunized by subcutaneous injections of 50 μg of the protein dimer described in Example 1 and a control monomer, respectively (monomer: murine PrP23-231 verus dimer: murine PrP23-231=23-231 [connected by the linker sequence], both with the 3F4 epitope and an N-terminal poly-histidine tag). At day 0 the proteins were combined with Freund's Complete Adjuvant, for boostering (at day 21 and day 42, respectively) the proteins were combined with Freund's Incomplete Adjuvant. Ten days after the last immunization blood samples for antibody evaluation were taken. It could be shown that the application of the prion protein monomer (control) resulted in the generation of a lower amount of specific antibodies compared to the application of the prion protein diner. After application of the dimer about 50% of mice showed a pronounced accumulation of specific antibodies (FIG. 3 a). The induced specific polyclonal immune response was measured in immunoblot. Recombinant murine PrP23-231 was separated on SDS-PAGE, transferred to PVDF membranes and incubated in an immunoblot analysis with serial dilutions of sera of immunized mice (primary antibody). The pre-immune sera were used to eliminate background signals. Staining was accomplished using conjugated secondary antibodies (anti-mouse) and the enhanced chemiluminescence kit (ECL plus, Amersham Corp., Buckinghamshire, U.K.). The mice immunized with PrP dimer showed higher antibody titers. Dilutions beyond 1:3000 still resulted in detectable signals. This finding indicates that the used immunogen was capable of overcoming the auto-tolerance phenomenon known for species-identical prion proteins. Further data showed that the induced antibodies were not directed against the N-terminal fusion domain or the linker sequence being reactive against authentic murine PrP23-231.

A similar result could already be achieved using rabbits, i.e. the rabbits immunized with the prion protein dimer showed a stronger immune response compared to the rabbits immunized with the prion protein monomer (FIG. 3 b). Recombinant monomeric and dimeric prion proteins (PrPM and PrPD, respectively, which were used also as immunogens in immunization) were separated on SDS-PAGE, transferred to PVDF membranes and analyzed in immunoblot. The left panel shows incubation with the serum from the rabbit immunized with PrP monomer, the right panel from the arbbit immunized with PrP dimer. Staining was accomplished using conjugated anti-rabbit secondary antibodies and the ECL plus kit. Whereas the left panel shows mainly a reaction against the monomeric PrP, the right panel demonstrates a much stronger immune reactivity which is directed against monomeric and dimeric prion protein.

EXAMPLE 3 The Application of Antibodies Generated by Immunization with a prion protein dimer Leads to Inhibition of the de novo Synthesis of PrP^(Sc) in vitro

The biological relevance of the immunization experiments described in Example 2 could be verified by the following in vitro experiment. ScMHM2 cells [cultured murine neuroblastoma cells (ATCC CCL 131); persistently infected with RML prions; stably transfected with a 3F4-tagged PrP; c.f. Schätzl et al., J. Virol. 71, 8821-8831, (1997)] which are infected with the infectious version of the prion protein (PrP^(Sc)) and permanently produce PrP^(Sc) were incubated with different types of antibodies. Incubation was carried using (a) commercially available monoclonal antibodies (3F4) (Signet Pathology Systems, Dedham, Mass., USA) which are directed against an epitope of the prion protein produced in ScMHM2 cells and (b) the antiserum obtained from rabbits immunized with the prion protein dimer described in Example 1. During incubation with antibodies for 16 hours proteins were metabolically labeled using ³⁵S-methionine and subsequently immunoprecipitated (RIPA). Prior to the RIPA the cell lysates were separated into two fractions for digestion with proteinase K (+/−) and separated into soluble and insoluble fractions using ultracentrifugation (1 hour, 100,000×g, 1% sarcosyl). As shown in FIG. 4, after incubation with the prion protein dimer for 16 hours a drastic inhibition of the de novo synthesis of PrP^(Sc) could be observed. The results observed with the 3FA antibody are less pronounced. It can be concluded that this result is due to generation of a functional PrP^(c) knock out state on the cell surface and, accordingly, an inhibition of the generation of PrP^(Sc). These results demonstrate in living cells that the induced anti-PrP antibodies are capable of binding to surface-located authentic PrP^(c) under native conditions (proof of principle for interaction with surface-located PrP under native conditions). Even more, bound antibodies are apparently internalised with PrP^(c) demonstrating a high binding affinity (resulting in an insoluble PrP of full length which is PK sensitive, see full-length signal in FIG. 4A, lanes 5-8). Finally, endogeneous PrP^(Sc) biogenesis was heavily impaired resulting in no more de novo generated PK resistant PrP^(Sc) (FIG. 4B, lanes 7 and 8). The biological effect on PrP^(Sc) biogenesis is therefore demonstrated as well (proof of principle for impairment of prion biogenesis). The efficiency of this principle could also be shown by using particular chemical compounds: The functional removal of surface-PrP^(c) resulted in a reduction of the production of prion proteins. 

1. A prion protein dimer, wherein the prion protein dimer is a homodimer comprising two prion protein monomers derived from the same species or a heterodimer comprising two prion protein monomers derived from different species, wherein the prion protein dimer is a fusion protein, and wherein each of the prion protein monomers comprise a full-length prion protein monomer or a truncated prion protein monomer having at least 20 amino acids thereof.
 2. The prion protein dimer of claim 1, wherein the prion protein monomers of the fusion protein are linked via a peptide spacer.
 3. The prion protein dimer of claim 2, wherein the peptide spacer is (a) a spacer having a length of 2-3 amino acids, wherein the DNA sequence encoding the spacer contains a restriction site, (b) an oligomer composed of small neutral amino acids, or (c) an oligomer having an amino acid sequence that defines an epitope.
 4. A prion protein dimer, wherein the prion protein dimer is a homodimer comprising two prion protein monomers derived from the same species or a heterodimer comprising two prion protein monomers derived from different species, wherein each of the prion protein monomers comprise a full-length prion protein monomer or a truncated prion protein monomer having at least 20 amino acids thereof and wherein the prion protein monomers are covalently coupled via polyethyleneglycol, activated benzodiazepine, oxazolone, azalactone, diketopiperazine or a monosaccharide.
 5. The prion protein dimer of claim 1, wherein each of the prion protein monomers comprise a native full-length prion protein monomer or an N-terminally and/or C-terminally truncated prion protein monomer having at least 20 amino acids thereof.
 6. A immunogenic composition comprising (i) a covalently coupled prion protein dimer, wherein the prion protein dimer is a homodimer comprising two prion protein monomers derived from the same species or a heterodimer comprising two prion protein monomers derived from different species, wherein each of the prion protein monomers comprise a native full-length prion protein monomer; (ii) an adjuvant; and (iii) a pharmaceutically acceptable carrier therefor.
 7. The composition of claim 6, wherein the prion protein dimer is purified.
 8. The composition of claim 6, wherein the prion protein dimer is a homodimer comprising two native full-length prion protein monomers.
 9. The prion protein dimer of claim 1, wherein the prion protein dimer is a homodimer comprising two native full-length prion protein monomers.
 10. The composition of claim 6, wherein the prion protein dimer is a fusion protein. 